1. Field of the Invention
The present invention relates to a dual band radio telephone for receiving and transmitting of radio signals in a first and a second frequency band, and to a method of receiving and transmitting of such radio signals. Such a radio telephone can be a dual band cellular or satellite phone, or the like. The radio signals can carry full duplex radio telephony signals, such as in a dual band AMPS/CDMA system, or quasi full duplex radio telephony signals, such as in a GSM/IDCS1800 system, or a mix of full duplex and quasi full duplex signals. Typically, in dual band cellular telephony, frequencies in the first and second frequency bands are separated by a 1:2 ratio, GSM operating a 900 MHz band and DCS1800 operating in an 1800 MHz band.
2. Description of the Related Art
In the U.S. Pat. No. 5,768,691, antennas and antenna circuitry for a dual band radio telephone are disclosed. Disclosed are various circuits for coupling first and second transceivers of the dual band radio telephone to transmit/receive antennas, such as coupling of an internal dual band, dual feed point transmit/receive antenna to the first transceiver through a first duplexer or frequency splitter, and to the second transceiver through a first duplexer. Such a circuit, comprising duplexers, is typically meant for full duplex radio telephony signals. Another antenna coupling circuit is disclosed, typically meant for quasi fill duplex systems in which there is no simultaneous transmission and reception, with transmit/receive switches instead of duplexers. Both duplexers and transmit/receive switches exhibit insertion losses. A duplexer typically causes transmitter output losses of up to 3 dB so that the overall transmitter efficiency is reduced, or, stated differently, battery energy from a radio telephone battery is wasted so that a radio telephone talk time is reduced.
Other transmit/receive antenna and circuit configurations in radio telephones are known. In one type of a dual band mobile phone, such as an AMPS/CDMA radio telephone, a dual band transmit/receive whip antenna is coupled to a dual band diplexer followed by transmit/receive duplexers for the respective frequency bands. In another type of radio telephone with a shared dual band whip antenna, for TDMA or GSM systems, transmit/receive switches are used instead of duplexers. In still other type of dual band mobile phones, a single feed point of an embedded dual band transmit/receive antenna, i.e., an antenna internal a housing of the radio telephone, is coupled to respective transmit/receive duplexers, or to respective transmit/receive switches, of respective transceivers comprised in the radio telephone through a diplexer, the duplexers being applied in a dual band AMPS/CDMA phone, and the switches being applied in a TDMA/GSM phone.
In the article xe2x80x9cDual-band antenna for hand held portable telephonesxe2x80x9d, Z. D. Liu and P. S. Hall, ELECTRONICS LETTERS, Mar. 28, 1996, VoL 32, No. 7, pages 609-610, a dual band, dual feed point transmit/receive antenna is disclosed. The disclosed antenna is a so-called planar dual-band inverted F-antenna exhibiting two different resonance frequencies, at 900 MEz and at 1800 MHz. When such an antenna is applied in a dual band radio telephone for full duplex telephony signals, such as an AMPS/CDMA phone, in accordance with the analog US cellular and TIA IS-95 Standards, or for quasi duplex telephony signals, such as a TDMA/JGSM phone, in accordance with the TIA IS-136 and GSM Standards, a diplexer can be dispensed with, but duplexers or antenna switches are still needed.
In the U.S. Pat. No. 5,828,348, a dual-band octafilar helix antenna is disclosed, particularly suitable for satellite communication.
So, dual band radio telephones are known at least comprising transmit/receive duplexers or transmit/receive switches, such duplexers or switches exhibiting undesirable insertion losses.
In the handbook, xe2x80x9cRF and Microwave Circuit Design for Wireless Commnicationsxe2x80x9d, L. E. Larson, Artech House, Inc., 1996, pages 45-51, the phenomenon of the occurrence of image rejection frequencies is described. It is well-known in the art that such undesired image frequencies can be rejected by image reject mixers, by image reject band pass filter, or by a combination thereof, in conjunction with duplexers if the case may be.
It is an object of the invention to provide a dual band radio telephone with reduced insertion losses.
It is another object of the invention to comply with image frequency rejection requirements in a dual band radio telephone.
It is still another object of the invention to comply with transmit-receive separation requirements in a dual band radio telephone.
In accordance with the invention, a dual band radio telephone is provided comprising: a first transceiver with a first receive branch and a first transmit branch, said first transceiver being operable in a first frequency band;
a second transceiver with a second receive branch and a second transmit branch, said second transceiver being operable in a second frequency band;
a dual band receive antenna comprising a first receive radiator and first receive feed point coupled to said first receive radiator, and a second receive radiator and a second receive feed point coupled to said second receive radiator, said first and second receive radiators being operable in said first and second frequency bands, respectively; and
a dual band transmit antenna comprising a first transmit radiator and first transmit feed point coupled to said transmit radiator, and a second transmit radiator and a second transmit feed point coupled to said second transmit radiator, said first and second transmit radiators being operable in said first and second frequency bands, respectively,
said first and second receive feed points being coupled to said first and second receive branches, respectively, and said first and second transmit feed points being coupled to said first and second transmit branches, respectively.
An advantage is that, because of reduced receive and transmit losses, a radio telephone battery is exhausted at a slower pace so that the radio telephone has a longer talk time. This is because removal of a duplexer achieves a better power efficiency of the transmitter amplifier, the transmitter amplifier consuming a major part of the radio telephone battery""s energy.
In order to fill image frequency rejection requirements, image reject band pass filters are provided between the receive feed points in the respective receive branches and inputs of the respective low noise amplifiers in the radio telephone, for at least partly rejecting undesired image frequencies in the respective frequency bands. As such, the occurrence of image frequencies, and image reject band pass filters to reject such image frequencies are well-known in the art. Further image frequency rejection can be done in image reject band pass filters following the low noise amplifiers, and even still further by image reject mixers for down-converting the received radio signals.
Although in principle no output filtering is required of output signals from the transmit power amplifiers, because there is no galvanic path between the transmit power amplifier outputs and the receive branches, and thus, in principle, no natural path loss, some filtering might be required when the dual band, dual feed point receive antenna and the dual band, dual feed point transmit antenna are placed at a relatively close distance internal the housing of the radio telephone. But such an additional power amplifier output filtering will exhibit much less insertion loss as compared to insertion losses due to a duplexer. Typically, the transmit portion of a duplexer gives rise to an insertion loss of some 3 dB, only 1 dB or so insertion loss being caused by such additional transmit power amplifier filtering.